An L-mode guided wave is generated, for example, by a configuration in FIG. 1. In FIG. 1, a coil 21 is wound around an inspection target 1 (pipe in the diagram), and a magnet 23 is attached to the inspection target 1. If an alternating current is passed through the coil 21 in this state, an alternating-current magnetic field is generated. By use of magnetic force resulting from this alternating-current magnetic field and a magnetic field resulting from the magnet 23, the inspection target 1 which is a ferromagnetic body is vibrated by a magnetostrictive effect, and a guided wave which is a kind of a sound wave is thereby generated. The generated guided wave propagates through the inspection target 1 along its longitudinal direction.
A reflected wave of the guided wave is detected to inspect the soundness of the inspection target 1. The guided wave is reflected as a reflected wave by a discontinuous part in the inspection target 1 and by a change in the cross-sectional area of the inspection target 1. By detecting the reflected wave, the soundness of the inspection target 1 is inspected. Regarding the soundness of the inspection target 1, for example, the inspection target 1 is inspected for defects such as a scarred or corroded part.
As compared with a sound wave used in general sound wave inspections, the guided wave attenuates less and enables the soundness of the inspection target 1 to be inspected over a wide range of the inspection target 1. The sound wave used in general sound wave inspections has, for example, a high frequency of 5 MHz and a small wavelength of 0.6 mm, and accordingly easily attenuates. In contrast, the above-mentioned guided wave has, for example, a low frequency of several kHz to several ten kHz and a great wavelength of 100 mm, and accordingly does not easily attenuate.
For example, as prior art of the present application, there is Patent Literature (PTL) 1 specified below.